The present invention relates to a photopolymerizable printing plate having a dimensionally stable support, a photopolymerizable layer, a top layer located thereon, and a protective sheet, said top layer comprising two mutually incompatible elastomeric block copolymers. It further relates to a process for producing such a photopolymerizable printing plate, to a process for producing relief printing plates, especially flexographic printing plates, and also to relief printing plates having ink cells in the relief surface.
Photopolymerizable printing plates are fundamentally known and generally feature a photopolymerizable layer which comprises a polymeric binder, a photopolymerizable compound, and a photopolymerization initiator or initiator system, with or without further customary constituents. For the production of flexographic printing plates preference is usually given to elastomeric binders such as block copolymers, examples being triblock copolymers with styrene-isoprene-styrene blocks or styrene-butadiene-styrene blocks, as described for example in DE-A 22 15 090. Printing plates comprising these elastomers as binders are soft and elastic and are therefore highly suitable for use in flexographic printing.
The printing properties are critically influenced by the surface of the printing plate. Parameters such as roughness, abrasiveness, surface tension, surface tack and solvent resistance have a great influence on ink transfer and the impression characteristics. It is known to construct printing plates from a plurality of layers, especially two layers. The two-layer construction of a printing plate, comprising a photopolymerizable layer and a relatively thin top layer located thereon, has the advantage that the properties of the surface of the printing plate can be modified without affecting the typical flexo properties of the printing plate such as, for example, hardness or elasticity. Surface properties and layer properties can therefore be modified independently of one another in order to achieve the optimum printing result.
EP-A 084 851 discloses a process for producing a photopolymerizable printing plate having a top layer. First of all, a so-called cover element is produced by applying the top layer to a cover film by casting or extrusion. Thereafter, a hot, extruded photopolymer composition is passed into the nip of a calender and calendered between a support and the cover element, so forming a photopolymerizable layer between them. The top layer comprises an elastomeric binder and optionally a second, nonelastomeric binder, especially a methyl methacrylate-acrylonitrile-butadiene-styrene tetrapolymer.
EP-A 456 336 discloses a photopolymerizable printing element having a top layer comprising as binder two elastomeric polymers: a crosslinking block copolymer and one which can be an acrylonitrile-butadiene or acrylonitrile-isoprene copolymer, carboxylated acrylonitrile, or a polyacrylate.
DE-A 40 22 978 discloses a photopolymerizable printing plate having a disadhesive top layer comprising a radial (polystyrene/polybutadiene)4Si block copolymer and optionally, further, a methyl methacrylate-acrylonitrile-butadiene-styrene tetrapolymer. The top layer formed in this way is intended to prevent the image mask from sticking fast to the surface of the plate during exposure.
On exposure of the photopolymerizable printing plates, the acrylonitrile-containing polymers are incorporated by crosslinking in the form of small beads into the top layer, so that the surface of the printing relief of the polymerized and developed printing plate is populated with small beads.
Following exposure, the printing plates described are developed using chlorinated solvents such as tetrachloroethylene or mixtures of tetrachloroethylene and n-butanol. On ecological grounds, however, it is now common to use nonchlorine wash solutions based on high-boiling hydrocarbon solvents, as disclosed by EP-A 470 071, for example. However, the polymers containing acrylonitrile and methacrylate are largely insoluble in such solvents, with the result that polymer sludge is deposited in the washers, a consequence of which is increased cleaning effort.
In the interests of economic printing plate production, it is also required that the properties of the surface of a printing plate be able to be modified in a controlled and simple manner without the need for any great quantity or complexity of apparatus, in order to be able to tailor the surface individually for the particular intended application.
It is an object of the present invention to provide a photopolymerizable printing plate which has a top layer and can readily be developed using nonchlorine wash solutions. A further object is to provide a printing plate whose surface can be tailored individually and simply to the particular intended application yet is tack-free and exhibits outstanding ink acceptance.
We have found that these objects are achieved by a photopolymerizable printing plate having a dimensionally stable layer support, a photopolymerizable layer and an elastomeric top layer applied thereto, and a protective sheet.
In the printing plate of the invention the elastomeric top layer comprises
40-95% by weight of a crosslinkable elastomeric block copolymer A comprising vinylaromatic blocks and diene blocks, and
5-50% by weight of a noncrosslinkable elastomeric block copolymer B comprising vinylaromatic blocks and alkylene blocks,
said block copolymers A and B being mutally incompatible.
The invention also provides a relief printing plate having ink cells in the surface of the printing relief, said relief printing plate being preparable from a photopolymerizable printing plate of the type specified above. In general, said ink cells have a diameter and a depth of not more than 15 xcexcm, preferably not more than 10 xcexcm.
The relief printing plates of the invention having ink cells in the surface feature a tack-free surface and excellent ink acceptance. Their surface properties, especially their roughness, can be very simply and optimally adapted to the particular intended application.
The invention is also illustrated by the attached figures. FIGS. 1 to 3 show electron micrographs of individual screen dots of different flexographic printing plates.
FIG. 1 shows a flexographic printing plate without a top layer.
FIG. 2 shows a flexographic printing plate with a top layer, in accordance with the prior art, produced from a photopolymeric printing element having a top layer comprising an SIS block copolymer and a methyl methacrylate-acrylonitrile-butadiene-styrene tetrapolymer. The flexographic printing plate has beads in the surface.
FIG. 3 shows a flexographic printing plate having a top layer in accordance with the invention, produced from a photopolymeric printing element having a top layer comprising a crosslinking SIS block copolymer and a noncrosslinking SEBS block copolymer. The flexographic printing plate has ink cells in the surface.
FIG. 4 shows a section through the top layer of a photopolymerizable printing plate of the invention with domains of B in a continuous phase of A (scale 260:1).